A well used to produce fluids, such as oil, gas, or water, preferably is “completed” by placing completion hardware or tools permanently or semi-permanently in the wellbore. The completion hardware or tools may include, for example, valves, packers, screens, and various other devices designed to protect the well or assist in the recovery of the fluids.
A typical subterranean well includes a casing string that lines the wellbore wall. The casing string is generally installed by running it into the wellbore and cementing it in place. The cementing process typically includes pumping a desired volume of cement into a central passageway of the casing string. Once the desired volume of cement has been pumped, a different fluid such as drilling fluid or “mud”, is pumped into the central passageway of the casing string, causing the cement to be displaced from the central passageway and into the annular region between the wellbore wall and the casing string. The cement sets in the annulus and bonds the casing string to the wellbore wall. The well may also have a liner and the liner is also cemented in the wellbore. Liner differs from casing in that a liner is hung from the bottom of a casing or another liner and not connected to the surface (as is casing).
Once the wellbore is cased, the formation fluids are sealed off from the interior of the casing or liner. In those zones adjacent reservoir rocks having desirable formation fluids, the casing is perforated to establish fluid communication between the formation fluids and the interior of the casing or liner so those fluids can be produced. This is typically done using a perforating gun. The perforating gun is lowered into the wellbore to a desired depth and, upon command, fires shaped charges radially outward through the casing or liner and into the formation, forming holes in the casing or liner and perforation tunnels in the formation.